1. Field of the Invention
The present invention relates to key telephone apparatus, and in particular it relates to a key telephone central office line circuit which reduces the number of connecting leads required for signalling between the line circuit and the associated telephone station sets. Still more particularly, the invention relates to control circuitry for each central office line using a total of only three leads: two talking lines and one control line. The control circuitry is used in common for all telephone sets connected to the associated central office line and performs such functions as incoming call detection, hold detection, and automatic hold release in response to central office line disconnection.
2. Description of Prior Art
In prior art key telephone apparatus, the control of each central office line connected to a plurality of key telephone station sets was performed with four leads: two talking leads, one control lead and one lamp lead. Each central office line required these four leads and as the number of central office lines contained in a key telephone system increased, the number of leads used increased accordingly. This increased manufacturing costs and was unsightly in appearance.
In recent years the telephone equipment market has experienced a very strong demand for natural resource convervation, cost reduction, functional improvement and better appearance. Consequently, the present invention was created in a key telephone apparatus market atmosphere that called for the removal of the deficiencies present in prior art key telephone apparatus. It is therefore an object of this invention to provide a new key telephone system line circuit which would achieve the same functions as prior art key telephone line circuits with only three conductors per central office line instead of four.
It is a further object of this invention to provide a key telephone line circuit which lowers the cost of the control circuitry by combining the circuits for the central office line incoming call detection function and the hold detection function.
It is also an object of this invention to provide an economical automatic hold release function under control of the central office line by combining the circuits for the central office line incoming call detection function and the hold function.